This invention relates generally to apparatus for indicating the bearing of RF signals emanating from an unknown position and particularly to an antennae system which adaptively determines the direction of arrival of RF signals by selectively steering antenna pattern nulls toward the source of the signal.
Direction finding systems have been employed for some time to determine the direction of arrival of an RF signal having an unknown source position. One method and apparatus utilized in such a system is a large aperture single antenna which produces a narrow beam pattern. Rotating the antenna causes the beam pattern to rotate through space and the direction of arrival of the RF signal can be located within the angular width of the antenna beam. The requirement of a large aperture, however, limits the frequencies at which this technique can be used and, generally, a large, thin array antenna system having 10 or more elements is employed for low frequency reception at less than one GHz. By comparing the phase of the RF signal at each of the antenna elements, bearing information can thus be obtained. Because of the requirement for a physically large antenna system, a correspondingly large mounting platform is required. In the case of mobile platforms such as aircraft or land vehicles or in any situation where the size of the platform must be restricted, a large antenna system may not be efficiently employed. In the VHF frequency range, the antenna aperture required to produce accurate bearing information becomes unrealistically large for installation on most aircraft. One direction finding system used in locating sonobuoys requires the adaptation of a 10 element antenna array to a limited number of aircraft that are large enough to efficiently support such an array, but such a system cannot efficiently be adapted to smaller aircraft platforms such as helicopters.